1. Field of the Invention
The invention relates to an improved method of isomerizing fatty acids which contain an isolated diene structure (CH=CH--CH.sub.2 --CH=CH--) and/or disproportionating conjugated diene rosin acids by bringing them in contact with sulfur and/or iodine at an elevated temperature.
2. Description of the Prior Art
An alkali soap of rosin acids and/or fatty acids is quite commonly used as a polymerization emulsifier when producing synthetic elastomers by free radical polymerization. The use of a fatty acid soap in addition to a rosin soap provides certain advantages, such as an acceleration of the polymerization.
Certain structural requirements are set for a fatty acid suitable for use as a polymerization emulsifier (1). Saturated fatty acids, as well as those containing one double bond, can be used. Thus, stearic and oleic acids are commonly used for this purpose. On the other hand, diene and polyene fatty acids, which contain the structural group --CH=CH--CH.sub.2 --CH=CH--, are not suitable to be used as emulsifiers owing to their retarding effect on the polymerization reaction. Fatty acids which contain the conjugated diene structure --CH=CH--CH=CH-- do not have this disturbing influence, while the fatty acids which contain a conjugated triene structure are not suitable for use as polymerization emulsifiers.
Tall oil fatty acid obtained from crude tall oil by distillation and which usually has a fatty acid content of over 90 % cannot be used as such as a polymerization emulsifier of synthetic elastomers. For example, a typical Finnish tall oil fatty acid contains 50-60 % fatty acids which contain the structure --CH=CH--CH.sub.2 --CH=CH-- and inhibit polymerization (linoleic and cis-5,9,12-octadecatrienoic acids). Some known methods for eliminating the acids of the said type from tall oil fatty acid are hydrogenation, dimerization, and isomerization. Technical hydrogenation and dimerization require a pressure reaction. The most common of the known isomerization methods (2) is most likely heating with an alkali, in which case the major part of the polyunsaturated fatty acids present in the tall oil fatty acid become conjugated. However, the technical realization of the method presents difficulties and the method has not become widely used.
The disproportionated rosin which is used as a polymerization emulsifier of synthetic rubber has traditionally been made mainly from gum rosin with the help of noble metal catalysts. Some other known disproportionation/dehydrogenation catalysts of rosin are nickel, sulfur, selenium, and iodine.
German Patent Specification No. 1,205,530 describes the disproportionation of gum rosin in two stages with the help of sulfur (1-10 %) and iodine (0.5 -2 %). The optimal rates are sulfur 2.8-3.5 % and iodine 0.8-1.0 %. The obtained product cannot be used as such but must be decolorized by a chemical treatment, e.g., with oxalic acid in mineral spirit. To obtain the final product, the solvent must be removed by distillation. Simultaneous use of sulfur and iodine yields a valueless pitch-like product.
U.S. Patent No. 3,277,072 describes the production of so-called fluid rosin with an iodine catalyst from tall oil rosin. Iodine is used at a rate of 0.01-3 % of the amount of rosin. The obtained product is characterized by a softening point (ball and ring method) below 55.degree.C and a dehydroabietic acid content of less than 40 %. A complete elimination of the abietic acid from tall oil rosin by means of iodine is difficult without a considerable simultaneous decarboxylation of the rosin acids.
One of the objects of the present invention is to provide a method of producing from Finnish tall oil rosin a product which can be used as a polymerization emulsifier of synthetic rubber, in which the total amount of rosin acids with a conjugated diene system is 0-2 % and the dehydroabietic acid content calculated from the total rosin acids is over 50 %, and from which the poly-unsaturated fatty acids containing the structural group --CH=CH--CH.sub.2 --CH=CH-- can be eliminated; these fatty acids are originally present in the rosin and are typical of Scandinavian crude tall oil. An additional object is to create a simple production process (no use of a solvent and no redistillation of rosin). Furthermore, the rosin should be light enough as such, without chemical decolorization. The softening point of the rosin should be above 55.degree.C to make transportation and handling in containers possible during summer. An additional requirement was that the product be odorless and have the lowest possible sulfur content.